Combustion of Siloxanes forms Silicates
The table below shows the various silicon-containing species found in biogas. AFT has analyzed the gas from over 400 landfills and municipal digesters and found siloxanes and/or other silicon-bearing organics in all of them. In order to demonstrate how destructive siloxanes can be, the pounds and Kg per hour and year of silicates (as SiO2--the main constituent of sand) have been calculated based on combustion of only 1 ppmv in the gas stream (the average biogas stream concentrations are higher than 1 ppmv). Not all of the silicates produced by combustion are expelled with the exhaust gases. Silicate material remaining in the combustion chambers forms a deposit on heated surfaces and abrades moving parts. These silicates are directly responsible for overheating, energy losses, reduction in operating hours, increased maintenance costs and sometimes, equipment failure.
| Species | Molecular Weight | % Silicon | Gas Concentration | Silicates Generated (as SiO2) | ||||
| ppm v/v | mg/m3 | lb/Hr | Kg/H | lb/Yr | Kg/Yr | |||
| Tetramethylsilane | 88.2 | 31.84 | 1.0 | 3.66 | 0.0013 | 0.0006 | 11.4 | 5.2 |
| Trimethylsilanol | 90.2 | 31.14 | 1.0 | 3.74 | 0.0013 | 0.0006 | 11.4 | 5.2 |
| Tetramethyldisiloxane | 134.3 | 41.82 | 1.0 | 5.57 | 0.0026 | 0.0012 | 22.9 | 10.4 |
| Pentamethyldisiloxane | 148.4 | 37.87 | 1.0 | 6.15 | 0.0026 | 0.0012 | 22.9 | 10.4 |
| MM (L2) | 162.4 | 34.60 | 1.0 | 6.73 | 0.0026 | 0.0012 | 22.9 | 10.4 |
| Hexamethyltrisiloxane | 208.5 | 40.42 | 1.0 | 8.64 | 0.0039 | 0.0018 | 34.3 | 15.6 |
| MDM (L3) | 236.5 | 35.63 | 1.0 | 9.80 | 0.0039 | 0.0018 | 34.3 | 15.6 |
| MD2M (L4) | 310.7 | 36.17 | 1.0 | 12.88 | 0.0052 | 0.0024 | 45.7 | 20.7 |
| MD3M (L5) | 384.9 | 36.50 | 1.0 | 15.95 | 0.0065 | 0.0030 | 57.2 | 25.9 |
| D3 | 222.5 | 37.88 | 1.0 | 9.22 | 0.0039 | 0.0018 | 34.3 | 15.6 |
| D4 | 296.6 | 37.88 | 1.0 | 12.29 | 0.0052 | 0.0024 | 45.7 | 20.7 |
| D5 | 370.8 | 37.88 | 1.0 | 15.37 | 0.0065 | 0.0030 | 57.2 | 25.9 |
| D6 | 444.9 | 37.88 | 1.0 | 18.44 | 0.0078 | 0.0036 | 68.6 | 31.1 |
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Conditions: Silicates generated based on 140 SCFM (238 Nm3H) for a 1 MW Generator Engine |
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Analysis of a Silicate Deposit
Below is the analysis of a deposit that was taken from an SCR catalyst. This deposit is primarily silicate originating from siloxanes in the fuel gas.
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Silicon |
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42.3% |
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Oxygen |
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33.3% |
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Calcium |
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18.5% |
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Sulfur |
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3.1% |
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Iron |
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2.8% |
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